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6. Notice of an Observation of the Spectrum of a Solar Prominence†
... Knowledge that helium [2,3] was first observed in the Sun by Pierre Jules César Janssen [4] and Joseph Norman Lockyer [5], before being discovered on Earth by William Ramsay [6], might prompt the belief that the element was abundant on the solar surface. In fact, helium has never been identified in the absorption spectra of the quiet Sun. ...
... In fact, helium has never been identified in the absorption spectra of the quiet Sun. Janssen and Lockyer's fortunate discovery was restricted to helium lines appearing within the prominences of the corona and within the disturbed chromosphere [4,5]. While the element was easily detectable in these regions [7], helium has remained relatively spectroscopically silent on the Sun. ...
Before a solar model becomes viable in astrophysics, one must consider how the ele-mental constitution of the Sun was ascertained, especially relative to its principle com-ponents: hydrogen and helium. Liquid metallic hydrogen has been proposed as a solar structural material for models based on condensed matter (e.g. Robitaille P.-M. Liq-uid Metallic Hydrogen: A Building Block for the Liquid Sun. Progr. Phys., 2011, v. 3, 60–74). There can be little doubt that hydrogen plays a dominant role in the uni-verse and in the stars; the massive abundance of hydrogen in the Sun was established long ago. Today, it can be demonstrated that the near isointense nature of the Sun's Balmer lines provides strong confirmatory evidence for a distinct solar surface. The situation relative to helium remains less conclusive. Still, helium occupies a prominent role in astronomy, both as an element associated with cosmology and as a byproduct of nuclear energy generation, though its abundances within the Sun cannot be reliably estimated using theoretical approaches. With respect to the determination of helium lev-els, the element remains spectroscopically silent at the level of the photosphere. While helium can be monitored with ease in the chromosphere and the prominences of the corona using spectroscopic methods, these measures are highly variable and responsive to elevated solar activity and nuclear fragmentation. Direct assays of the solar winds are currently viewed as incapable of providing definitive information regarding solar helium abundances. As a result, insight relative to helium remains strictly based on the-oretical estimates which couple helioseismological approaches to metrics derived from solar models. Despite their "state of the art" nature, helium estimates based on solar models and helioseismology are suspect on several fronts, including their reliance on solar opacities. The best knowledge can only come from the solar winds which, though highly variable, provide a wealth of data. Evaluations of primordial helium levels based on 1) the spectroscopic study of H-II regions and 2) microwave anisotropy data, re-main highly questionable. Current helium levels, both within the stars (Robitaille J. C. and Robitaille P.-M. Liquid Metallic Hydrogen III. Intercalation and Lattice Exclusion versus Gravitational Settling, and Their Consequences Relative to Internal Structure, Surface Activity, and Solar Winds in the Sun. Progr. Phys., 2013, v. 2, in press) and the universe at large, appear to be overstated. A careful consideration of available ob-servational data suggests that helium abundances are considerably lower than currently believed. At the age of five Cecilia [Payne] saw a meteor, and thereupon decided to become an Astronomer. She remarked that she must begin quickly, in case there should be no research left when she grew up. Betty Grierson Leaf, 1923 [1, p. 72–73]
... Though the Sun would always remain devoid of the great advantage of our earthly laboratories, it has historically provided us with an amazing insight into nature. When Sir Joseph Lockyer and Pierre Jules César Janssen independently observed the lines of helium within solar spectra acquired in 1868 [126][127][128][129][130] , they must have wondered if this unknown element would ever be discovered. Lockyer named this element H¯ elios, the Greek name for the Sun god and the Sun [126]. ...
The establishment by Andrews of critical temperatures (T. Andrews, Phil.
Trans. 1869, v. 159, 575-590) soon became one of the great pillars in
support of the gaseous models of the Sun. Gases above these temperatures
simply could not be liquefied. Given that interior of the Sun was
already hypothesized in the 19th century to be at temperatures well
exceeding those achievable on Earth in ordinary furnaces, it became
inconceivable to think of the solar interior as anything but gaseous.
Hence, the models advanced by Secchi, Faye, Stoney, Lane, and Young,
could easily gain acceptance. However, modern science is beginning to
demonstrate that hydrogen (which under ordinary conditions has a
critical point at ˜33 K) can become pressure ionized such that its
electrons enter metallic conductions bands, given sufficiently elevated
pressures, as the band gap is reduced from 15 eV to ˜0.3 eV.
Liquid metallic hydrogen will possess a new critical temperature well
above that of ordinary hydrogen. Already, experiments suggests that it
can exist at temperatures of thousands of Kelvin and millions of
atmospheres (S. T. Weir et al., Phys. Rev. Let. 1996, 76, 1860). The
formation of liquid metallic hydrogen brings with it a new candidate for
the interior of the Sun and the stars. Its existence shatters the great
pillar of the gaseous models of the Sun which the critical point of
ordinary gases had erected.
Henri Becquerel, while searching for X-rays, discovers a radiation emitted by uranium. The scientific community shows no interest in such a weak and incomprehensible phenomenon with no practical applications.
Solar prominences are one of the most common features of the solar atmosphere. They are found in the corona but they are one hundred times cooler and denser than the coronal material, indicating that they are thermally and pressure isolated from the surrounding environment. Because of these properties they appear at the limb as bright features when observed in the optical or the EUV cool lines. On the disk they appear darker than their background, indicating the presence of a plasma absorption process (in this case they are called filaments). Prominence plasma is embedded in a magnetic environment that lies above magnetic inversion lines, denoted a filament channel.
This paper aims at providing the reader with the main elements that characterize these peculiar structures, the prominences and their environment, as deduced from observations. The aim is also to point out and discuss open questions on prominence existence, stability and disappearance.
The review starts with a general introduction of these features and the instruments used for their observation. Section 2 presents the large scale properties, including filament morphology, thermodynamical parameters, magnetic fields, and the properties of the surrounding coronal cavity, all in stable conditions. Section 3 is dedicated to small-scale observational properties, from both the morphological and dynamical points of view. Section 4 introduces observational aspects during prominence formation, while Section 5 reviews the sources of instability leading to prominence disappearance or eruption. Conclusions and perspectives are given in Section 6.
The chromosphere and corona of the Sun represent tenuous regions which are characterized by numerous optically thin emission lines in the ultraviolet and X-ray bands. When observed from the center of the solar disk outward, these emission lines experience modest brightening as the limb is approached. The intensity of many ultraviolet and X-ray emission lines nearly doubles when observation is extended just beyond the edge of the disk. These findings indicate that the solar body is opaque in this frequency range and that an approximately two fold greater region of the solar atmosphere is being sampled outside the limb. These observations provide strong support for the presence of a distinct solar surface. Therefore, the behavior of the emission lines in this frequency range constitutes the twenty fifth line of evidence that the Sun is comprised of condensed matter.
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